ROMK (Kir1.1) forms apical K channels in the distal nephron and collecting duct that provide the K secretory pathways for 1) K recycling necessary for NaCI absorption by the thick ascending limb and 2) K secretion by connecting duct and principal cells. Therefore, defining the function and regulation of the ROMK channels is important for understanding renal K handling and body K homeostasis. Loss-of-function mutations in ROMK cause Bartter's syndrome, a hypotensive renal salt wasting disease, that is due to loss of salt reabsorbing capacity by the thick ascending limb. We have developed the only ROMK Bartter's mouse with sufficiently high survival (>30%) to permit physiological study - and is the only mouse model of Bartter's with such high survival. We propose to continue to study the pathophysiology of mouse ROMK Bartter's to enhance our understanding of the role of ROMK in renal K handling and to suggest potential modalities for therapy of human Bartter's syndrome. We will also develop two new ROMK transgenic mice that will provide models for assessing both the specific roles of ROMK isoforms and the regulated trafficking of ROMK isoforms in native kidney cells. One of these isoforms, ROMK1, is only expressed in late distal tubule and collecting duct and has been suggested to play an important role in the regulation of renal K secretion by dietary K intake. We propose to generate a mouse deficient only in ROMK1 by selective deletion of the ROMK1-specific exon using a Cre-LoxP strategy. We hypothesize that this mouse will not have a Bartter's phenotype, but instead, have disordered K handling with increases or decreases in dietary K intake. We will also generate a ROMK1 BAC-transgenic mouse tagged with FLAG and EGFP to define ROMK trafficking in kidney epithelial cells and to determine associated proteins in the ROMK regulatory complex. These mice will also provide a resource for others interested in ROMK function in non- renal cells (Gl tract, brain, etc).